Platelets are essential components of arterial thrombosis. Interference with platelet signal transduction can dramatically reduce morbidity and mortality of platelet-mediated thrombotic disease such as heart attack, stroke, and peripheral vascular disease. Identification of optimal therapeutic targets for inhibition of platelet activation has been slowed, however, because platelets are anucleate and not amenable to genetic manipulations frequently used to study signal transduction mechanisms in nucleated cells. In contrast, the ready availability of platelets and their ability to undergo profound phenotype change upon exposure to agonists has made them a popular cellular model for studies using small molecules. We have developed a high throughput assay to identify novel anti-platelet agents that inhibit platelet activation as monitored by dense granule secretion. The goal of this undertaking is to discover new molecular targets that control platelet granule secretion. In collaboration with a facility within the Molecular Library Screening Center Network (MLSCN), we will expand the number of compounds that we screen, improve the rate at which screening is performed, and optimize compounds with antiplatelet activity. We will evaluate granule secretion in two related assays. One will identify compounds that inhibit dense granule secretion induced by SFLLRN, which stimulates platelets via protease-activated receptor 1. The second assay will identify compounds that inhibit dense granule secretion induced by collagen I, which activates platelets via glycoprotein VI. This strategy will facilitate identification of compounds that inhibit platelet function by diverse mechanisms. Once identified, compounds will be tested in a previously established battery of platelet function tests and evaluated for antithrombotic potential in an in vivo murine model of thrombus formation. [unreadable] [unreadable] [unreadable]